Creating pre-completed Tasks

Danny Shih


We’ve been considering adding support for creating completed Tasks from an existing result.  Here’s a prototypical example of where this could be valuable.


void Task<float> ComputeAsync(…)
    if (!resultIsCached)
        return Task<float>.Factory.StartNew(() => Compute());
        // return a Task<float> with the cached result

The method usually returns a Task<float> that represents some compute-intensive operation that will be done asynchronously.  However, the greater code has the ability to cache results from previous operations, so there’s a chance that the requested result is already available.  If that’s the case, we just want to return a completed Task with the cached result.  Note that this can be done in .NET 4 as follows:


TaskCompletionSource<float> tcs = new TaskCompletionSource<float>();
return tcs.Task;

But we could make this easier and slightly better-performing:


return Task.FromResult(cachedResult);

So your input would help.  If you’ve got a minute, feel free to answer the following questions and/or provide any other thoughts you have:

  1. Would the convenience make this feature worthwhile?
  2. If you have code that resembles this example, is performance a huge concern (to the point that shaving a few allocations and interlocked operations off of the creation of a pre-completed Task would help)?
  3. What about support for creating other pre-completed Tasks, e.g. creating a pre-canceled Task or one from an existing Exception?  Details regarding the scenario would help here.




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